Goal:
  
  z : Z
  z0 : Z
  k : t
  H : Some (Z.max z z0) = k
  H1 : (z < z0)%Z
  H2 : Z.max z z0 = z0
  ============================
   (Some z = k \/ Some z0 = k)


DEBUG:Going to execute:
fun => <coq-core.plugins.ltac::left@0>